Preliminary studies of aqueous extracts of bovine testes and ram rete testis fluid demonstrate a dose dependent suppression of FSH and LH in an in-vitro bioassay utilizing LRF stimulated gonatropin release from neonatal rat pituitary cell primary cultures. This gonatropin suppression does not appear to be due to sex steroid contamination nor degradation of LRF. These data are encouraging for the hypothesis that a non-steroid FSH inhibitor (inhibin) is produced by the seminiferous tubule.